It happens all the time;

https://www.usnews.com/news/national-news/articles/2019-07-0....

> Now, increasingly, more frequent heat waves and hotter average temperatures are making those waters so warm that engineers are concerned that it can't do the job. Analysts say climate change is to blame.

> In little-noticed but publicly available reports to regulators, nuclear plant owners revealed that unusually hot temperatures last year forced them to reduce the plants' electricity output more than 30 times – most often in the summer, when demand from nuclear plants is at its highest. In 2012, such incidents occurred at least 60 times. At one plant in Connecticut a reactor was taken offline for nearly two weeks when temperatures in the Long Island Sound surged past 75 degrees.

It's not like you can just change the legislation to say "allow hotter discharge water" though -- the plants' very engineering criteria is being impacted. Warmer water has lower absorptive capacity to cool the primary loop so things get dicey when temps increase outside the design specification.

All designs that take water from rivers and return it there. There usually is a regulation for the maximum water temperature in the rivers, so during the summer they can't heat up the water as much, leading to lowered energy output to the point where they have to shutdown some or all reactors of a plant.

That doesn’t imply that nuclear plants regularly get close to this threshold nor that it’s common for all nuclear plants of this type (consider 2 identical plants positioned on differently-sized rivers: the larger river will be able to absorb more heat than the smaller river, ergo reliability of this sort varies).

It's not just the river's ability to absorb the heat, it's also the plant's design criteria. The river water is the coolant in the secondary loop, warm water obviously has less capacity to absorb heat, so the plant will either get hotter (not great!) or will have to slow down the reaction to produce less heat (also not great during the summer when demand is the highest!). This isn't just an environmental concern but a design problem.

Not OP but I google dit and found this [0] from almost 20 years ago. No mention of it being every summer though, and it was during a heat wave that disproportionately affected france.

[0] https://www.theguardian.com/world/2003/aug/12/france.nuclear